By combining data from the vast online archives from many of the world's premier telescopes, astronomers have finally identified one of the nearest supernovae of the last 25 years, over a decade after it exploded.

SN 1996cr was first singled out by Franz Bauer in 2001 who noticed a bright, variable source in the Circinus spiral galaxy while using NASA's Chandra X-ray Observatory. Although the source displayed some exceptional properties its nature could not confidently be identified.

This composite image shows the central regions of the nearby Circinus galaxy, located about 12 million light years away. Data from NASA's Chandra X-ray Observatory is shown in blue and data from the Hubble Space telescope is shown in yellow ("I-band"), red (hydrogen emission), cyan ("V-band") and light blue (oxygen emission). The bright, blue source near the lower right hand corner of the image is the supernova SN 1996cr, that has finally been identified over a decade after it exploded. Image: X-ray (NASA/ CXC/Columbia/F.Bauer et al); Optical (NASA/STScI/UMD/A.Wilson et al.)

Years later, clues that the mystery object was a supernova came from a spectrum obtained by ESO’s Very Large Telescope. This prompted Bauer and colleagues to start a campaign of searching through data from 18 different ground and space based telescopes, to see what further clues they could find. Fortunately the nearby Circinus galaxy is a popular target for astronomers because it contains a supermassive black hole that is actively growing, and it shows vigorous star formation, so the public archives of these telescopes contained a wealth of information, revealing SN 1996cr as one of the brightest supernovae ever seen in radio and X-rays. It also bears many striking similarities to the famous supernova SN 1987A, which occurred in a neighbouring galaxy only 160,000 light-years from Earth.

"This supernova appears to be a wild cousin of SN 1987A," says Bauer. "The two look alike in many ways, except this newer supernova is intrinsically a thousand times brighter in radio and X-rays."

Visible-light images from the archives of the Anglo-Australian
Telescope in Australia show that SN 1996cr exploded sometime between 28 February 1995 and 15 March 1996, but it is the only one of the five nearest supernovae of the last 25 years that was not seen shortly after the explosion. But since it was first detected by Chandra in 2001, it has become steadily brighter. Previously, SN 1987A was the only known supernova with an X-ray output that increased over time.

"It's a bit of a coup to find SN 1996cr like this, and we could never have nailed it without the serendipitous data taken by all of these telescopes,” says Bauer. “We've truly entered a new era of 'internet astronomy’.”

By combining the plethora of observational data with theoretical work, the team have now developed a model for the explosion. They suggest that before the parent star even exploded, it cleared out a large cavity in the surrounding gas, either via a strong wind or from an outburst from the star late in its life, allowing the blast wave from the explosion to expand relatively unimpeded into the cavity. Once the blast wave hit the dense material surrounding SN1996cr, the impact caused the system to glow brightly in X-ray and radio emission.

Astronomers think that both SN 1987A and SN 1996cr show evidence for these pre-explosion clear-outs by a star doomed to explode. Having two nearby examples suggests that this type of activity could be relatively common during the death of massive stars.

"Not only does our work suggest that SN 1987A isn't as unusual as previously thought, but it also teaches us more about the tremendous upheavals that massive stars can undergo over their lifetimes," says co-author Vikram Dwarkadas of the University of Chicago.

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